fix(docs-infra): use a fixed top-menu on marketing pages as well (#41051)
fix(docs-infra): use a fixed top-menu on marketing pages as well (#41051)

Previously, in contrast to docs pages, marketing pages had a non-fixed
top-menu, which meant that the top-menu would scroll out of the viewport
along with the rest of the content. This had a couple of downsides:
- The UI was different between pages (i.e. different top-menu behavior
  on docs vs marketing pages).
- Since some of the marketing pages are long, it was not easy for people
  to navigate to a different page (i.e. they had to scroll all the way
  back up).

This commit improves the UX by using the same, fixed top-menu on all
pages, which restores consistency and allows the user to navigate around
more easily.

NOTE:
The old behavior (non-fixed top-menu) is kept on the homepage, since its
top-menu design in a little different than other pages (e.g. it uses a
transparent top-menu) and would not play well with a fixed top-menu.

PR Close #41051

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